bottom cylinder fouling Big Twin 25hp

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    The red boat

    US Member - 1 Year
    Replies: 52
    Topics: 18
    #219302

    Ok – I’m getting stumped on an issue I’m having with my ’55 Evinrude 25hp powerhead.
    The motor has been hard starting recently but once it starts, it’s good to go for the rest of the day. idles perfectly; runs strong all day.

    i recently swapped out the spark plugs (to NGK’s for better starting) and somehow stripped the lower plug hole so I installed a NOS cylinder head and new gasket. While doing this i inspected the cylinders and made sure the rings weren’t stuck. it looked really clean and nice inside the cylinders.

    I went to start the motor yesterday for the first time since the new head was installed and i’m severely fouling the bottom plug. I pulled the head this morning and you can see the carbon on the piston (that wasn’t there before).
    Yesterday when i finally got it running, i pulled the plug wires one at a time to make sure it was running on both cylinders and it was. fuel delivery seemed fine.

    to me it seems like the bottom cylinder isn’t getting spark (or enough spark) and building up carbon from unspent fuel very quickly. I thought maybe i had a broken ring which is why i pulled the head this morning but they checked out ok when inspecting them thru the bypass cover.

    i have coils that are 2 years old and Atom electronic ignition modules that have never given me any grief. new plug wires too.
    compression is 90 to 95 on each cylinder.

    any thoughts?

    IMG_5773

    • This topic was modified 1 month ago by MumblesMumbles.
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    fleetwin

    US Member - 2 Years
    Replies: 3530
    Topics: 46
    #219341

    Kind of hard to say…. Did you decarbonize the pistons while changing the head? What does the other piston look like? What do the plugs look like? You seem to indicate that the engine runs just fine, except that it is hard to start cold. What is it that got you to pull the new head back off?

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    lyks2tinkr

    US Member - 2 Years
    Replies: 356
    Topics: 44
    #219347

    Just a guess but wondering if the head gasket was weeping water into the cylinder causing incomplete combustion.

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    The red boat

    US Member - 1 Year
    Replies: 52
    Topics: 18
    #219351

    The pistons didn’t need decarb when I swapped out the head. They were pretty clean with the normal amount of darkening. The top piston still looks like that.
    The bottom plug was clean initially but after running like crap for about 5 minutes I pulled them. The bottom was completely black and somewhat dry and the top one was tan and just slightly damp (oil).
    I pulled the new head back off because I thought maybe I screwed up the gasket or something since it wasn’t fouling plugs before. Gasket looked fine but tore when I removed the head – waiting for another one now.

    Buccaneer
    Buccaneer

    US Member - 1 Year
    Replies: 4169
    Topics: 922
    #219352

    What about the “oil drain system” and that little flopper check valve in the front crankcase cover.
    If that’s not working will you get excess fuel and oil in the bottom cylinder, causing
    your symptoms?

    Prepare to be boarded!

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    fleetwin

    US Member - 2 Years
    Replies: 3530
    Topics: 46
    #219353

    What about the “oil drain system” and that little flopper check valve in the front crankcase cover.
    If that’s not working will you get excess fuel and oil in the bottom cylinder, causing
    your symptoms?

    Good point…I guess my question is how did the engine run/plugs look before the head change?? If all was well prior to the change, then something changed with the new head….How do the combustion domes look in the new head, and irregularities?? And, like someone else mentioned, perhaps there is a water leak somewhere, but usually this causes the opposite issue, pistons domes cleaned up from water bath…..

    Mumbles
    Mumbles

    Canada Member - 2 Years
    Replies: 4550
    Topics: 353
    #219366

    Quite possibly you have weak spark at the bottom cylinder. Since the plugs are new, the problem might be with the spring connector inside the boot. Rust, corrosion, and old age can all cause a poor electrical connection here. Weak spark could also be caused by dirty points or a failed condenser, or an old coil starting to go South. With the Atoms installed, points or condensers won’t be an issue here.

    DSC00031-1

    DSC00086-6

    • This reply was modified 1 month ago by MumblesMumbles.
    • This reply was modified 1 month ago by MumblesMumbles.
    • This reply was modified 1 month ago by MumblesMumbles.
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    wrenchero

    US Member - 2 Years
    Replies: 68
    Topics: 32
    #219385

    just wondering if you converted this engine to a fuel pump. i had a similar problem that was caused by a fuel pump diaphragm that was slightly torn.just a thought

    mn

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    jeff-register


    Replies: 1235
    Topics: 50
    #219391

    Jim, I’m thinking the same thing with electronic type trigger. When I did research on the caps the rep said a mag is very hard on solid state parts. Tried looking up Ford transistor type switching with darlington type transistors failed too. Ford discintuned that ign so the transistors were hard to find too.
    I too would look at leaf seating on the reeds too or a piece of dirt holding the leaf plate from a 100% seating position. Has the compression changed during repair process too?

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    The red boat

    US Member - 1 Year
    Replies: 52
    Topics: 18
    #219418

    thanks for all the replies.
    I’ll try to answer best i can.
    – the compression is the same as it was before
    – the cylinders and pistons looked pretty clean when i changed the head – normal darkening but no carbon build-up
    – i too thought about the plug wires and connections since it was becoming very difficult to start even before the head swap. That is what i am going to check next although my plug wires and boots were replaced 2 years ago. perhaps the bottom one is loose.
    – I do believe the bottom cylinder has a weaker spark because when i pulled plug wires when it was running there was a significant difference between the 2 cylinders and how smooth they ran individually.
    – no fuel pump on this one; just the pressurized tank
    – i haven’t checked the oil drain system – will check that also. what is the fix for that? new assembly or just clean?

    i have cleaned the new head and pistons; checked the rings to make sure they aren’t frozen; replaced a dirty inline plastic fuel filter;
    now i am waiting for a new head gasket but in the meantime I will start with the plug wires and go from there.
    the thing ran great all summer and only became hard starting in the last month or so – prior to the new head. so something has changed without me being the cause of the foul-up 🙂

    thanks again

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    crosbyman

    Canada Member - 1 Year
    Replies: 1721
    Topics: 204
    #219421

    just curious … what have you measured in ohms value….. boot clip to engine frame ….both should be similar and steady if you wiggle the boots or wires

    Joining AOMCI has priviledges 🙂

    Mumbles
    Mumbles

    Canada Member - 2 Years
    Replies: 4550
    Topics: 353
    #219425

    Something to think about when using an ohmmeter to test plug leads, the meter can show continuity through the wire indicating it is good. But, if the internal core is broken and still joined by a single strand or there is a poor connection under the boot, the the meter will indicate the wire is good when in fact it won’t be able to carry the high voltage needed to fire the plug. Believe me, I’ve been stumped on this before.

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    crosbyman

    Canada Member - 1 Year
    Replies: 1721
    Topics: 204
    #219428

    agree… the best option in this case is to recheck/redoo … replace all the suspect electrical components if any doubt exist on the quality of wires, boots, coil connections etc… at least it would eliminate that part of the puzzle (fuel oxygen…. fire)

    the arrival problem coincides with the new cyl head presumably it was resurfaced and installed properly so the external variable here are the wires or plugs . (unless Murphy showed up in the meantime)

    a bad plug interchange could move the problem over to the top cyl … end of problem 🙂

    the next step is a close look over of the lower cyl HV circuit all the way back to the coil HV pin

    jmho

    Joining AOMCI has priviledges 🙂

    • This reply was modified 1 month ago by Avatarcrosbyman.
    • This reply was modified 1 month ago by Avatarcrosbyman.
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    The red boat

    US Member - 1 Year
    Replies: 52
    Topics: 18
    #220542

    I have been out of town and unable to work on this motor but I plan to do the following this weekend:
    1. check crankcase drain
    2. check coils with ohmeter
    3. check continuity of plug wires. Likely will replace with new wires I have on-hand.
    4. check all wiring under the flywheel; solder and re-connect
    5. install new spark plugs
    6. I have spare Atom modules – I might swap out the one for the lower cylinder with a new one just to be safe
    7. verify ignition-throttle synchro
    8. remove sintered element in carb fuel bow (I have installed an inline plastic replaceable fuel filter and can’t tell if the bowl filter is clear or not).
    9. examine high and low speed needles.

    any other thoughts on this list or the order?

    Mumbles
    Mumbles

    Canada Member - 2 Years
    Replies: 4550
    Topics: 353
    #220544

    #3 above is probably your best option.

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